The inherent flexibility afforded by molecular design has accelerated the development of a wide variety of organic semiconductors over the past two decades. In particular, great advances have been made in the development of materials for organic light-emitting diodes (OLEDs), from early devices based on fluorescent molecules to those using phosphorescent molecules. In OLEDs, electrically injected charge carriers recombine to form singlet and triplet excitons in a 1:3 ratio; the use of phosphorescent metal-organic complexes exploits the normally non-radiative triplet excitons and so enhances the overall electroluminescence efficiency. Here we report a class of metal-free organic electroluminescent molecules in which the energy gap between the singlet and triplet excited states is minimized by design, thereby promoting highly efficient spin up-conversion from non-radiative triplet states to radiative singlet states while maintaining high radiative decay rates, of more than 10(6) decays per second. In other words, these molecules harness both singlet and triplet excitons for light emission through fluorescence decay channels, leading to an intrinsic fluorescence efficiency in excess of 90 per cent and a very high external electroluminescence efficiency, of more than 19 per cent, which is comparable to that achieved in high-efficiency phosphorescence-based OLEDs.

Efficient thermally activated delayed fluorescence (TADF) has been characterized for a carbazole/sulfone derivative in both solutions and doped films. A pure blue organic light emitting diode (OLED) based on this compound demonstrates a very high external quantum efficiency (EQE) of nearly 10% at low current density. Because TADF only occurs in a bipolar system where donor and acceptor centered (3)ππ* states are close to or higher than the triplet intramolecular charge transfer ((3)CT) state, control of the π-conjugation length of both donor and acceptor is considered to be as important as breaking the π-conjugation between them in blue TADF material design.

Organic compounds that exhibit highly efficient, stable blue emission are required to realize inexpensive organic light-emitting diodes for future displays and lighting applications. Here, we define the design rules for increasing the electroluminescence efficiency of blue-emitting organic molecules that exhibit thermally activated delayed fluorescence. We show that a large delocalization of the highest occupied molecular orbital and lowest unoccupied molecular orbital in these charge-transfer compounds enhances the rate of radiative decay considerably by inducing a large oscillator strength even when there is a small overlap between the two wavefunctions. A compound based on our design principles exhibited a high rate of fluorescence decay and efficient up-conversion of triplet excitons into singlet excited states, leading to both photoluminescence and internal electroluminescence quantum yields of nearly 100%.

Efficient organic light-emitting diodes have been developed using emitters containing rare metals, such as platinum and iridium complexes. However, there is an urgent need to develop emitters composed of more abundant materials. Here we show a thermally activated delayed fluorescence material for organic light-emitting diodes, which realizes both approximately 100% photoluminescence quantum yield and approximately 100% up-conversion of the triplet to singlet excited state. The material contains electron-donating diphenylaminocarbazole and electron-accepting triphenyltriazine moieties. The typical trade-off between effective emission and triplet-to-singlet up-conversion is overcome by fine-tuning the highest occupied molecular orbital and lowest unoccupied molecular orbital distributions. The nearly zero singlet–triplet energy gap, smaller than the thermal energy at room temperature, results in an organic light-emitting diode with external quantum efficiency of 29.6%. An external quantum efficiency of 41.5% is obtained when using an out-coupling sheet. The external quantum efficiency is 30.7% even at a high luminance of 3,000 cd m−2.

Efficient thermally activated delayed fluorescence (TADF) was developed in a material based on a phenoxazine (PXZ) electron donor unit and a 2,4,6-triphenyl-1,3,5-triazine (TRZ) electron acceptor unit. An organic light-emitting diode containing this novel TADF emitter layer was fabricated and exhibited a maximum external quantum efficiency of 12.5% with green emission.

A material containing a phenothiazine (PTZ) electron donor unit and 2,4,6-triphenyl-1,3,5-triazine (TRZ) electron acceptor unit, PTZ-TRZ, which exhibits thermally activated delayed fluorescence (TADF) was developed. Density functional theory calculations revealed the existence of two ground-state conformers with different energy gaps between the lowest singlet excited state and lowest triplet excited state (1.14 and 0.18 eV), which resulted from the distortion of PTZ, as confirmed by X-ray structure analysis. PTZ-TRZ in toluene solution showed two broad, structureless emissions, confirming the existence of two different excited states. From detailed analyses of the absorption and photoluminescence spectra, we determined that both emissions were intramolecular charge-transfer (ICT) fluorescence. Therefore, the excited-state conformers of PTZ-TRZ resulted in dual ICT fluorescence. Because previously reported dual fluorescence from single molecules involves locally excited and ICT fluorescence, the dual ICT fluorescence from PTZ-TRZ is novel. Temperaturedependence of transient PL spectra of a 2 wt % PTZ-TRZ-doped film in 3,3′-bis(N-carbazolyl)-1,1′-biphenyl measured by a streak camera revealed that the former and latter emissions were independent of and dependent on the film temperature, respectively. This confirms that the dual fluorescence involves TADF characteristics. An organic light-emitting diode containing PTZ-TRZ exhibited a maximum external quantum efficiency of 10.8 ± 0.5% with dual ICT fluorescence.

Emission wavelength tuning of thermally activated delayed fluorescence from green to orange in solid state films is demonstrated. Emission tuning occurs by stabilization of the intramolecular charge transfer state between a phenoxazine (PXZ) donor unit and 2,4,6-triphenyl-1,3,5-triazine (TRZ) acceptor unit separated by a large twist angle. The emission wavelengths of mono-, bis-, and tri-PXZsubstituted TRZ exhibit a gradual red shift while maintaining a small energy gap between the singlet and triplet excited states. An organic light-emitting diode containing a tri-PXZ-TRZ emitter exhibited a maximum external quantum efficiency of 13.3 ± 0.5% with yellow-orange emission. KEYWORDS: organic light-emitting diodes, thermally activated delayed fluorescence, emission wavelength tuning, intramolecular charge transfer ■ INTRODUCTIONSince our group reported the first observation of electroluminescence (EL) based on thermally activated delayed fluorescence (TADF) from a Sn 4+ −porphyrin complex, 1 the potential of TADF materials as emitters for organic lightemitting diodes (OLEDs) has been revealed. 2 A remarkable feature of TADF is up-conversion of excitons from the lowest triplet excited state (T 1 ) of a compound to its lowest singlet excited state (S 1 ), which strongly depends on the energy gap between them (ΔE S-T ). Up-conversion from T 1 to S 1 can be realized in molecules with donor−acceptor (D−A) moieties that induce intramolecular charge transfer (ICT). TADF materials are currently attracting considerable attention as third-generation OLEDs because of their high EL efficiency and lack of rare metals such as Ir and Pt. To replace the present OLEDs based on fluorescent and phosphorescent materials, methodology for RGB emission wavelength tuning is essential. The emission process of general fluorescent materials involves π−π* transitions via singlet excitons, and emission wavelength tuning is achieved by controlling the length of π-conjugation. Attaching substituents such as electron-donating or -withdrawing groups to fluorescent molecules is also an effective way to tune emission wavelength. 3 Conversely, the emission process of typical phosphorescent materials such as iridium complexes is based on triplet metal-to-ligand charge transfer ( 3 MLCT) transitions. Their emission wavelength has also been tuned by extending the conjugation of the ligand. 4 In the case of TADF materials, the emission process is categorized as ICT transitions via triplet excitons. To minimize ΔE S-T for efficient up-conversion, a TADF molecule needs its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) to be effectively separated, which can be achieved using a twisted structure. Effective HOMO−LUMO separation induces the ICT transition from HOMO to LUMO. Regarding the molecular design of TADF materials, simple extension of the π-conjugated system results in a large ΔE S-T and weak ICT transition, which induces a locally excited state and dual fluorescence. 5 While efficient blue 2,6 and green 2,7 TAD...

We developed highly-efficient thermally activated delayed fluorescence (TADF) emitters containing 2,5-diphenyl-1,3,4-oxadiazole (OXD) or 3,4,5-triphenyl-4H-1,2,4-triazole (TAZ) electron acceptor and phenoxazine (PXZ) electron donor moieties. Oxadiazole-based compounds PXZ-OXD and 2PXZ-OXD showed green emission, while the triazole-based ones PXZ-TAZ and 2PXZ-TAZ exhibited sky-blue emission. In toluene solution, the donor-acceptor-donor-type molecules 2PXZ-OXD and 2PXZ-TAZ showed more efficient TADF and higher photoluminescence quantum yields (PLQYs) than the donoracceptor-type molecules PXZ-OXD and PXZ-TAZ. When doped into a host material, 2PXZ-OXD displayed a high PLQY of 87%. An organic light-emitting diode using 2PXZ-OXD as an emitter exhibited an external quantum efficiency (EQE) of 14.9%, which exceeds those obtained with conventional fluorescent emitters. This high EQE results from the efficient generation of TADF in 2PXZ-OXD.